1 files changed, 52 insertions, 0 deletions
diff --git a/9.py b/9.py
new file mode 100644
index 0000000..1e9270b
--- /dev/null
+++ b/9.py
@@ -0,0 +1,52 @@
+from dataclasses import dataclass
+
+@dataclass(frozen=True)
+class Point:
+    x: int
+    y: int
+    def __add__(self, other):
+        return Point(self.x + other.x, self.y + other.y)
+    # def __mul__(self, other):
+        # return Point(self.x * other, self.y + other)
+    def __sub__(self, other):
+        return Point(self.x - other.x, self.y - other.y)
+    def norm(self):
+        return Point(min(max(self.x, -1), 1), min(max(self.y, -1), 1))
+
+def pabs(p):
+    return max(abs(p.x), abs(p.y))
+delta = {
+    'U': Point(0, 1), 'D': Point(0, -1), 'L': Point(-1, 0), 'R': Point(1, 0)
+}
+with open('9.in') as f:
+    inp = [tuple(line.rstrip().split()) for line in f]
+inp = [(delta[d], int(q)) for d, q in inp]
+def solve(tails):
+    knots = [Point(0, 0) for _ in range(tails + 1)]
+    visited = set()
+    visited.add(knots[-1])
+    for move, count in inp:
+        for _ in range(count):
+            knots[0] += move
+            for i in range(1, len(knots)):
+                diff = knots[i-1] - knots[i]
+                if pabs(diff) > 1:
+                    knots[i] += diff.norm()
+            visited.add(knots[-1])
+        # for y in range(20, -20, -1):
+            # line = []
+            # for x in range(-20, 20):
+                # p = Point(x, y)
+                # if p in visited:
+                    # line.append('#')
+                # else:
+                    # line.append(' ')
+                # try:
+                    # line[-1] = str(knots.index(p))
+                # except ValueError:
+                    # pass
+            # print(''.join(line))
+    return len(visited)
+
+print(solve(1))
+print(solve(9))